Field of the Invention
The invention is concerned with the combined preparation of butene and octene from ethene.
Discussion of the Background
Hydrocarbons are chemical compounds which consist exclusively of carbon and hydrogen. Alkenes (synonym: Olefins) are hydrocarbons which have a C≡C double bond in the molecule. Alkanes (synonym: Paraffins), on the other hand, are hydrocarbons which have only single bonds. They are therefore also referred to as saturated.
In organic chemistry, hydrocarbons are frequently designated according to the number of carbon atoms which they have per molecule, by the respective class of substances being preceded by the prefix Cn. Here, n is the respective number of carbon atoms in a molecule. Thus, C4-olefins are substances from the class of alkenes having four carbon atoms. C8-olefins correspondingly have eight carbon atoms per molecule. Where the prefix Cn+ is used in the following, it refers to a class of substances which have more than n carbon atoms per molecule. A C4+-olefin accordingly has at least five carbon atoms.
The simplest olefin is ethene (ethylene). It has two carbon atoms. Ethene is an important basic chemical and is therefore prepared in large quantities. This is usually effected by steam cracking of naphtha. In addition, it can be obtained by dehydrogenation of ethane, which in turn is a constituent of natural gas. Owing to the increasing exploitation of unconventional sources of natural gas and decreasing recovery of petroleum, the proportion of ethene based on natural gas is steadily increasing.
C4-olefins encompass the four isomeric materials 1-butene, cis-2-butene, trans-2-butene and isobutene. 1-Butene and the two 2-butenes belong to the group of the linear butenes, while isobutene is a branched olefin. The linear C4-olefins 1-butene, cis-2-butene and trans-2-butene are often summarised as “n-butene” in the literature. Depending on the thermodynamic circumstances, the four isomeric C4-olefins usually occur together. For this reason, no distinction between singular and plural is made when the term “butene” is used. When reference is made here to “butene” with no further details being specified, what is meant is a linear alkene having four carbon atoms (or n-butene) or a mixture containing different isomeric alkenes having four carbon atoms.
A current overview of the chemical and physical properties of butenes and also the industrial processing and utilisation thereof is given by:                F. Geilen, G. Stochniol, S. Peitz and E. Schulte-Koerne: Butenes. Ullmann's Encyclopedia of Industrial Chemistry. (2013)        
Butenes are nowadays predominantly obtained in the cracking of petroleum fractions in a steam cracker or in a fluid catalytic cracker (FCC) and are used as intermediate for the preparation of a variety of industrial chemicals.
In the following, a “hexene” is an olefin having six carbon atoms or a mixture containing a plurality of different C6-olefins. For this reason, no distinction is made between singular and plural when using the term “hexene”. The C6-olefins include the eighteen isomers 1-hexene, (E)-2-hexene, (Z)-2-hexene, (E)-3-hexene, (Z)-3-hexene, 2-methyl-1-pentene, 2-methyl-2-pentene, (R)-3-methyl-1-pentene, (S)-3-methyl-1-pentene, (E)-3-methyl-2-pentene, (Z)-3-methyl-2-pentene, 4-methyl-1-pentene, (E)-4-methyl-2-pentene, (Z)-4-methyl-2-pentene, (3S)-2,3-dimethyl-1-butene, (3R)-2,3-dimethyl-1-butene, 2,3-dimethyl-2-butene and 3,3-dimethyl-1-butene.
However, only the substances 1-hexene and 4-methyl-1-pentene, which are used as monomers or comonomers in the production of plastics, are of industrial interest. For this purpose, they are prepared from ethene or from the C3-olefin propene by oligomerisation. The oligomerisation will be explained in detail below.
For the purposes of the present invention, octene is an olefin having eight carbon atoms or a mixture containing a plurality of different C8-olefins. The C8-olefins include a large number of isomers which are too many to list here. An industrially important representative of the C8-olefins is 1-octene which is prepared by oligomerisation of ethene and is used as comonomer in polyethylene.
An alternative way of preparing octene is dimerisation of n-butene. The mixture of olefins having eight carbon atoms which is formed here is referred to as dibutene, and is thus a particular octene within the meaning of the terminology employed here. Dibutene is distinguished by the isomer distribution, in terms of which it differs from other octene mixtures.
Depending on the way in which the individual n-butene molecules are joined in the course of the oligomerisation, an oligomer having a different degree of branching is obtained. The degree of branching is described by the iso index, which states the mean number of methyl groups per C8 molecule in the isomer mixture. The iso index for dibutene is defined as follows:Iso index=(proportion by weight of methylheptenes+2*proportion by weight of dimethylhexenes)/100
Thus, n-octenes contribute 0, methylheptenes contribute 1 and dimethylhexenes contribute 2 to the iso index of a product mixture of C8-olefins. The lower the iso index, the less branched are the molecules in the mixture.
A low degree of branching is always important when the olefin mixture is to be used as starting material for preparing plasticizers. Scientific studies demonstrate that the degree of branching of olefin mixtures which are processed further by hydroformylation, hydrogenation and esterification to give plasticizers is critical to the properties and quality of the plasticizer.
The iso index which a C8-olefin mixture has to achieve in order to be able to serve as starting material for high-quality plasticizers depends on the respective requirements of the plasticizer customers and changes over time. At present, an iso index of less than 1.1 is usually required.
For the purposes of the present invention the oligomerisation which has now been mentioned a number of times is the reaction of hydrocarbons with themselves, forming corresponding longer-chain hydrocarbons. Olefins having from two to eight carbon atoms can be oligomerised very readily.
Thus, for example, an olefin having six carbon atoms (hexene) can be formed by oligomerisation of two olefins having three carbon atoms. The oligomerisation of two molecules with one another is also referred to as dimerisation. If, in contrast, three olefins having three carbon atoms are joined to one another (trimerisation), the result is an olefin having nine carbon atoms. If n-butenes are subjected to an oligomerisation, essentially olefins having eight carbon atoms (more precisely: dibutene) and also olefins having twelve carbon atoms (C12-olefins, “tributene”) and to a lesser extent olefins having more than twelve carbon atoms (C12+-olefins) are formed.
One process employed in industry for preparing dibutene by oligomerisation of n-butene is the Octol® process. Detailed description thereof can be found in the nonpatent literature, for example in:                B. Scholz: The HÜLS Octol Process: Heterogeneously catalysed dimerisation of n-butenes and other olefins. DGMK conference in Karlsruhe, published in        Erdöl, Erdgas, Kohle, April 1989, pages 21 and 22.        R. H. Friedlander, D. J. Ward, F. Obenaus, F. Nierlich, J. Neumeister: Make plasticizer olefins via n-butene dimerisation. Hydrocarbon Processing, February 1986, pages 31 to 33.        F. Nierlich: Oligomerise for better gasoline. Hydrocarbon Processing, February 1992, pages 45 to 46.        
In the patent literature, an oligomerisation based on the Octol® process is described, for example, in DE102008007081A1. EP1029839A1 is concerned with the fractionation of the C8-olefins formed in the Octol® process.
The completely heterogeneously catalysed Octol® process gives a dibutene which has a low degree of branching and is highly suitable for the preparation of plasticizers. Heterogeneously catalysed means that the catalyst is present as a solid in the liquid or gaseous reaction mixture. The fluid reactants thus flow around the catalyst and the catalyst remains in the reactor.
The term cooligomerisation refers to the simultaneous oligomerisation of a plurality of substrates in one reaction vessel. Thus, EP2582648B1 describes the cooligomerisation of butene and octene to give dodecene (C12-olefin). As in the case of any oligomerisation, which olefin reacts with which is not precisely known in a cooligomerisation: In the example of EP2582648B1, a dodecene can be formed both from three butenes and also from a butene and an octene. From a chemical point of view, any oligomerisation can be considered to be a cooligomerisation. From an industrial point of view, on the other hand, a cooligomerisation is present only when at least two olefins having different numbers of carbon atoms are introduced into a common reactor. In the choice of terminology, it is thus the controllable introduction of the starter materials which matters, not the reaction which actually takes place.
WO2005/123884 discloses the combined preparation of 1-octene and 1-hexene by tetramerisation and trimerisation of ethylene. For this purpose, two different homogeneous catalysts, namely a first catalyst for tetramerisation and a second catalyst for trimerisation, are provided in a common reaction vessel. Since the homogeneous catalysts used are dissolved in the reaction mixture, they have to be either recycled with retention of their catalytic activity by suitable methods or be completely separated off. Recycling of a homogeneous catalyst is associated with complicated engineering and considerable costs in terms of apparatus, which appears to be feasible only in the case of very expensive catalysts. The complete separation of a homogeneous oligomerisation catalyst from a reaction mixture is usually effected by quenching with water or alkaline, aqueous solutions. This leads to significant generation of aqueous, often chromium-containing salt solutions which have to be disposed of appropriately. In addition, the use of fresh catalyst solutions for the oligomerisation incurs relatively high costs.
Furthermore, this process also does not appear to be suitable for preparing C8-olefins for use as starting material for plasticizers: Although up to 52% by weight of C8-olefins are obtained in combined tetramerisation and trimerisation, the degree of branching is not specified precisely. Moreover, the process is optimised for the production of the comonomer 1-octene, viz. a C8-olefin which in any case is not very suitable for plasticizer production. It is therefore not possible to see that the C8-alkenes achieve an iso index which qualifies them as starting material for plasticizer production. In addition, the homogeneously dissolved catalyst would definitely have to be separated off in this use since the subsequent hydroformylation is likewise homogeneously catalysed and is sensitive to interference caused by extraneous catalysts introduced by entrainment.
What has just been said also applies to the process disclosed in WO2005/123633 for the oligomerisation of ethylene, which is carried out in the presence of cyclohexane. The cyclohexane serves as solvent and is intended to reduce the deactivation of the homogeneous catalyst used or its activator.
A similar situation also applies to US2013/0066128 A1 which is concerned with the homogeneous oligomerisation of ethene in n-heptane.
The problem of separating off the catalyst does not arise in heterogeneously catalysed processes in which the catalyst is present as a solid and remains in the reactor. Ethylene oligomerisation over a solid Si/Al/Ni system is described in U.S. Pat. No. 8,637,722B2. However, this process takes place in the gas phase, which is disadvantageous in terms of the utilisation of space by the reactors. In addition, the established process steps of further processing of butenes and octenes take place in the liquid phase, so that this gas-phase process is not readily compatible with existing technology. A need to liquefy the butenes and octenes obtained in the gas phase requires additional energy.
The gas-phase process disclosed in WO2010/117539A1 for oligomerisation of ethylene diluted in an FCC gas over a zeolitic Ni catalyst also cannot be readily incorporated into an established production train for C4/C8 utilisation.
A mixed form of heterogeneous and homogeneous oligomerisation is disclosed in US2013/0158321A1. Here, ethene is firstly dimerised homogeneously to form butenes and these are subsequently converted into octenes by heterogeneous catalysis over a solid nickel catalyst. Both reaction stages take place in the liquid phase in the presence of hexane. The reaction output from the first stage has to be neutralised by means of base and freed of the homogeneous catalyst (triethylaluminium) by distillation. This is very complicated in industrial practice.